1. Field of the Invention
The present invention relates to a switching path setting system used in switching equipment for exchanging a fixed length cell, for example an ATM (Asynchronous Transfer Mode) switch, including a plurality of quality classes.
2. Description of the Related Art
It is required to send a variety of kinds of information such as voice, text data, moving pictures (or moving image data), in a communication system. In other words, a flexible switching system for exchanging a wide range of data transmission speeds is needed. An ATM system is one of these kinds of system.
In the ATM system, information is divided into fixed length (48 bytes) data. At the beginning of fixed length data, five bytes of control information (address information and the like) is added. This control information is referred to as a header. The resultant information block with a total of 53 bytes is referred to as a cell. In the ATM system, when information is transmitted at high speed, the number of cells transmitted per unit time is increased. When information is transmitted at low speed, the number of cells transmitted per unit time is decreased. In such a manner, the transmission speed of information can be easily adjusted. Thus, in the ATM system, a variety of services ranging from low speed information of the order of several kbps such as telephone voice data to high speed information of the order of around several 100 Mbps such as moving images can be integrally handled.
The routing of a cell is controlled by a VPI (Virtual Path Identifier) and a VCI (Virtual Channel Identifier) that are stored in the header thereof. In other words, when a cell is input to ATM switching equipment, it retrieves a VCI conversion table (VCCT) corresponding to the VPI/VCI of the cell. The VCI conversion table stores tag information including routing information in the ATM switching equipment and VPI/VCI added to an output cell corresponding to the VPI/VCI of the input cell. When a call is set, the content of the VCI conversion table is written. Thus, when a cell is input to the switching equipment, the VPI/VCI of the cell is rewritten using the VCI conversion table. In addition, the tag information that determines the routing in the switching equipment is added to the cell. The cell is routed in the switching equipment corresponding to the tag information and then output to a predetermined output line.
Since the ATM system integrally handles the above-mentioned various services, a plurality of data transmissions may be concentrated to a particular path and the sum of data transmission bands may exceed the capacity of the path. When the data transmissions exceed the capacity of the path, a transmission error or the like will take place. To prevent such a problem, ATM switching equipment is normally provided with a shaping function. The shaping function causes each cell to be temporarily stored in a buffer memory and the read speed of the cell from the buffer memory to be controlled so as to alleviate the periodical concentration of the traffic. As the construction of the buffer memory, there are for example cross-point type, input buffer type, output buffer type, and common buffer type. In the cross-point type, buffers are disposed at the cross-point of input and output of each switching element. In the input buffer type, a buffer is disposed at an input of the switching equipment. In the output buffer type, a buffer is disposed at an output of the switching equipment. In the common buffer type, a buffer is shared by all inputs or outputs of the switching equipment.
However, when a burst-like traffic such as data transmission in LAN is input, such a buffer memory will get instantaneously overflowed- In this case, the overflowed cells will be discarded.
The various services that the ATM provides require respective quality, such as quality for cell transmission time (absolute delay time and delay fluctuation) and quality for cell loss ratio (error ratio). In addition, each service requires different quality class. For example, when LAN data is transmitted by the ATM system, since it normally has data retransmission function that allows data to be retransmitted between terminals upon occurrence of a transmission error that takes place therebetween, even if a cell is discarded, it is retransmitted by the data retransmission function. Thus, in this case, the quality request for the cell loss ratio is not so strict. On the other hand, when image data is transmitted without such a data retransmission function, if high image quality is required, even a slight cell loss may adversely affect the image quality. Thus, in this case, the quality request for the cell loss ratio is very strict.
However, so far, characteristics of the path in the switching equipment have not been satisfactorily analyzed. If a cell is discarded due to for example burst-like data input, it is difficult to know in what service the cell was discarded. Thus, when the buffer gets overflowed, a cell whose quality request for the cell loss ratio is not strict may not be discarded. Instead, a cell whose quality request for the cell loss ratio is strict may be discarded.
Although the ATM switching equipment can process packet information corresponding to a frame-relay switching technique, this technique causes a path that is larger than a physical band to be allocated. Thus, if congestion takes place with packets corresponding to the frame-relay switching technique, it tends to adversely affect other ATM cells. Consequently, the ATM cells may be discarded, thereby deteriorating the quality of data transmission. In this case, it is difficult to know in what service ATM cells were discarded.
In the related art reference, priority and nonpriority concerning discarding cells are performed corresponding to a CLP (Cell Loss Priority) bit in the header of each of the cells. The CLP bit is a control bit where "0" and "1" represent priority cell and non-priority cell, respectively. When the buffer gets overflowed, the non-priority cells are discarded so as to prevent the priority cells from being discarded and maintain the quality of the priority cells. However, by the control method using the CLP bit, only two levels of priority are allocated. Thus, cell quality control for various services cannot be flexibly performed.
To solve such a problem, for example, when a call is set, a predetermined band of the switching equipment is allocated to the call corresponding to a declared band and a required quality. In this method, a predetermined band of the switching equipment should be allocated to each VPI/VCI of the call. However, since so many types of VPIs/VCIs are exchanged in the switching equipment, the table for allocating calls to bands becomes very large. In particular, when transmission speed becomes high, cells cannot be controlled corresponding to the transmission speed. In reality, when bands are controlled with buffers in the switching equipment, each buffer should be provided with a table that stores the relation between the VPIs/VCIs and allowable bands so as to compare the VPI/VCI of each cell with the VPI/VCI of the table. Thus, the construction for detecting and comparing the VPI/VCI becomes large and the process thereof becomes complicated.
As another method for solving such a problem, a plurality of switches are disposed in the switching equipment. Each service is allocated to each switch so as to prevent congestion of a path allocated to a particular service from adversely affecting other services. However, in this construction, the cost and size of the switching equipment increase. From this point of view, a path setting method that can prevent each service from adversely affecting other services, allow the quality of each service to be easily controlled, and use the existing switching equipment is required.